This invention relates to a coaxial engine starter in which an armature rotary shaft of an electric starter motor, an output rotary shaft having a pinion on it for rotation therewith and an operating axle of a solenoid switch are coaxially arranged.
FIG. 1 is a sectional view showing a coaxial engine starter to which the present invention can be applied. In FIG. 1, a coaxial engine starter 100 comprises a d.c. motor 5 having a magnetic yoke 1, permanent magnets 2 mounted to the inner surface of the magnetic yoke 1 at intervals, an armature 3 rotatably supported within the magnetic yoke 1, and a commutator 4 disposed at one end of the armature 3.
The armature 3 of the d.c. motor 5 comprises a hollow armature rotary shaft 6 and an armature core 7 mounted to the outer circumference of the armature rotary shaft 6. On the front end (the right-hand end in FIG. 1) of the d.c. motor 5, an output rotary shaft 8 is disposed so that the rotation is transmitted thereto through a drive force transmitting mechanism 9. The drive force transmitting mechanism 9 comprises a planetary speed reduction gear 10, an over-running clutch 11 having a clutch inner member 11a and a clutch outer member 11b, and a helical spline 8a formed on the output rotary shaft 8 for engaging with the clutch inner 11a. The output rotary shaft 8 is disposed in alignment with the axis of the armature rotary shaft 6 of the d.c. motor 5, and its one end is inserted into the inner passage 6a of the armature rotary shaft 6 and axially slidably supported therein by a sleeve metal inserted therebetween. In order to prevent a thrust from the output rotary shaft 8 from being transmitted into the armature rotary shaft 6, a gap A is provided between the armature rotary shaft 6 and an enlarged-diameter portion 8b of the output rotary shaft 8.
The transmission of the rotational force of the armature rotary shaft 6 is achieved through the planetary speed reduction gear 10 and the over-running clutch 11. The planetary speed reduction gear 10 comprises a sun gear 10a integrally formed on the front end of the armature rotary shaft 6, an inner teeth ring gear 10b disposed around the sun gear 10a and a plurality of planetary gears 10d rotatably supported by support pins 10c mounted to the clutch outer member 11b of the over-running clutch 11 and in engagement with the sun gear 10a and the inner teeth ring gear 10b. Also, the clutch inner member 11a of the over-running clutch 11 is in mesh with the helical spline 8a formed in the outer circumference of the enlarged diameter portion 8b of the output rotary shaft 8. Therefore, when output rotary shaft 8 is rotated by the clutch inner member 11a, it axially slides at the same time. Then, due to the sliding movement of the output rotary shaft 8, the pinion 15 mounted on the front end of the output rotary shaft 8 projects from the outer front bracket 13 to engage with the engine ring gear (not shown) to rotate it. A plurality of rollers 11c are disposed between the clutch inner member 11a and the clutch outer member 11b.
An inner teeth gear member 10e which also serves as an inner front bracket is fastened to the magnetic yoke 1 by bolts 34. The inner teeth gear member 10e has formed an inner teeth gear 10b in the inner circumferential surface at its rear end and a stepped portion 10f at its front end. The stepped portion 10f has fitted therein an outer race 12a of a ball bearing 12 with its rear end in an abutting relationship. The rear end of an inner race 12b of the bearing 12 is fitted onto a stepped portion 11d formed at the front end of the into a stepped portion 11d formed at the front end of the clutch inner member 11a. An outer front bracket 13 which may be replaced according to the type of the engine is fitted over the inner teeth gear member 10e which also is an inner bracket and fastened by bolts 14. The outer front bracket 13 also has an inner wall end portion 13a extending in front of the front end of the outer race 12a of the ball bearing 12 for bearing the forward thrust of the outer race 12a. A pinion engageable with an engine ring gear (not shown) which is mounted on the output rotary shaft 8 has a rear end face 15a in abutment with the inner race 12b of the ball bearing 12, so that its thrust force is supported by the inner race 12b of the ball bearing 12. In order to maintain the pinion 15 which is in a spline engagement with splines 8c formed on the output rotary shaft 8 at a predetermined axial position on the output rotary shaft 8, a stopper 16 is mounted on the output rotary shaft 8 by a stop ring.
A rear bracket 17 which is fitted over the rear end of the d.c. motor 5 has disposed in the rear portion thereof a solenoid switch 18 for shifting the output rotary shaft 8 and for allowing an electric power from a battery (not shown) to be supplied to the d.c. motor 5. The solenoid switch 18 comprises an excitation coil 21 wound on a plastic bobbin supported by a magnetic core 20 which together with a case 19 define a magnetic circuit, a plunger 22 slidably disposed within a central bore of the coil 21, a tubular rod 23 made of a nonmagnetic material such as stainless steel and connected at one end to the plunger 22 and inserted at the other end into an inner passage 6a of the armature rotary shaft 6 from the behind, and a movable contact 25 mounted on the rod 23 through an insulation 24. Within the tubular rod 23, a push rod 26 is slidably inserted so that the front end of the push rod 26 extending forward from the front open end of the tubular rod 23 abuts against the bottom wall of the recess formed in the end surface of the output rotary shaft 8 through a steel ball 27. A coil spring 28 is disposed within the tubular rod 23 to bias the push rod 26 and another coil spring 29 is disposed around the push rod 26 to hold the steel ball 27 in position. An electrically insulating material 30 is provided around a stationary contact 31 and an electric brush 33, the other end of the stationary contact 31 being a terminal bolt 32 for the connection of a cable to the unillustrated battery. The electromagnetic switch 18, the rear bracket 17, the d.c. electric motor 5 and the inner teeth gear member 10e are put together by the bolts 34.
The operation of the coaxial engine starter as above constructed will now be described. When the engine starter switch (not shown) is turned on, the solenoid switch 18 is energized to move forward the plunger 22 and the tubular rod 23, so that the output rotary shaft 8 is moved forward by the thrust transmitted through the coil spring 28 and the push rod 26. This causes the pinion 15 to engage the engine ring gear (not shown) and the movable contact 25 on the tubular rod 23 to contact with the stationary contact 31 to energize the d.c. motor 5. Then, the rotational force of the armature rotary shaft 6 of the d.c. motor 5 is transmitted to the clutch outer member 11b of the over-running clutch 11 through the planetary speed reduction gear 10, and this rotational force is further transmitted from the clutch inner member 11b to the output rotary shaft 8 to rotate the pinion 15, whereby the engine is driven.
After the engine is started and the power supply to the solenoid switch 18 is disconnected, the output rotary shaft 8 returns to its original position by a suitable return spring, thereby releasing the engagement between the pinion 15 and the engine ring gear (not shown). Also, the returned pinion 15 stops when its rear end surface 15a abuts against the front end of the inner race 12b of the ball bearing 12.
However, with the above-described coaxial engine starter, there is no shock-absorbing measure within the rotation transmission path extending from the armature rotary shaft 6 to the pinion 15, so that an excessive load is applied to the starter and the engine ring gear when an engine starting operation is carried out during the engine inertial rotation, leading to a fear that the components such as the pinion 15 of the starter can be damaged or destroyed.